Many successful demonstrations have been conducted on light non-aqueous phase liquid (LNAPL) removal and plume treatment technologies. Technologies such as bioventing, dual-phase extraction, air sparging and monitored natural attenuation (MNA) were demonstrated successfully to show that LNAPL could be removed and benzene, toluene, ethylbenzene, and xylenes (BTEX) plume migration could be controlled. The expectation was that, unlike chlorinated solvents, petroleum sites could be closed after a reasonable period of time. However, in the longer term, many of these sites are still struggling to remove enough free product to meet U.S. Environmental Protection Agency and state requirements of source control (product mobility) as well as plume treatment and are continuing to face long-term management challenges. The primary objective of this project was to help the Department of Defense (DoD) and others make a stronger case for closure of legacy petroleum sites, and expand users’ knowledge of high-impact methods that can better reveal the actual risk associated with LNAPL presence and therefore help stakeholders make more informed remediation decisions.

The field demonstration was completed at Naval Air Station Fallon located in Fallon, Nevada. Three methods were implemented to help assess the remaining LNAPL at a petroleum-impacted site and provide guidance for future application at other DoD sites. This included:

1. Applying a natural source zone depletion (NSZD) assessment using carbon traps to demonstrate that LNAPL bodies are not static, permanent contaminant sources, but are biodegrading at the rate of hundreds or thousands of gallons per acre per year.
2. Collecting soil, groundwater and LNAPL samples per the American Petroleum Institute’s Total Petroleum Hydrocarbon Criteria Working Group (TPH-CWG) residual risk methodology, applying the Washington State TPH fractionation method, and demonstrating that the risk associated with weathered hydrocarbons is significantly reduced over time compared to fresh hydrocarbons.
3. Using historical recovery data to calculate the transmissivity of the LNAPL body to demonstrate that there is no or very little risk of LNAPL migration.

This project assessed multiple methodologies to better characterize remaining LNAPL at petroleum-impacted sites and the evaluation of the residual risk remaining at these sites. These tools can be used as a supplement to help transition sites from active to passive recovery or even transition to site closure. Based on the results of this field investigation, active LNAPL recovery could potentially be discontinued without changing the low risk profile at the site. Costs were evaluated to determine what a similar investigation would cost to complete in order to assess the remaining LNAPL and determine a more cost-effective path forward for a site. As part of the cost assessment, transition to NSZD/MNA was compared with traditional free product recovery methods implemented for the recovery and management of LNAPL at impacted sites.

This project demonstrated three approaches for assessing residual LNAPL at petroleum-impacted sites after many years of various free product recovery efforts. The project identified best practices and guidance regarding the characterization and assessment of residual LNAPL in order to make more informed decisions regarding LNAPL remediation and site management strategies by refining the conceptual site model and evaluating any residual risk. Details on the implementation of the various methodologies/tools and specific considerations are included in the Supplemental Guidance document.